Field of the Invention
The present invention concerns a method and an apparatus for acquiring magnetic resonance (MR) signals from an examination object in an MR system that has a whole body coil and a local transmit coil.
Description of the Prior Art
In medical MR diagnostics, different methods are known for maximizing the signal quality and the contrast of the signal. These methods generally require the use of electromagnetic RF fields with a high field strength. The RF fields excite and refocus nuclear spins in the subject across the entire volume of the examination object in order to cause the spins to emit MR signals. A partial absorption of the radiated energy by the examination object, and thus a temperature elevation, occur in the process. A measure of this absorption or heating is the specific absorption rate (SAR). To prevent excessive heating, legal limit values exist for the SAR, which define which maximum energy output can be deposited in an object of a specific weight. To prevent these limit values from being exceeded, MR systems according to the current prior art make use of local volume coils for RF field generation for specific parts of an examination object. These fields are spatially limited and thus have a lower SAR. They are suited to body regions that can be completely surrounded by such a local volume coil. Examples are local volume coils for a knee or the head. Furthermore, local volume coils can achieve a reduction in the encoding time by reducing the measurement region from which the MR signals must be encoded, which can shorten the overall duration of the examination.
Even with special imaging, for instance in the case of implants, chemical-shift artifacts or spectroscopy, local transmit coils are advantageous because they can generate the highest possible amplitudes and bandwidths in a spatially restricted manner.
A notable drop in the field strength, however, frequently occurs at the edges of the field of the local volume coil.
If desired, imaging of the examination object encompasses body parts that cannot be enclosed, or can only be enclosed with difficulty, local volume coils cannot be used. Lower intensities for the refocusing pulse must therefore be accepted, which results in the quality of the image being impaired.
Known methods for maximizing the quality and the contrast of the MR signal require the highest possible RF field strengths, but this entails an increased heating of the examination object.
The use of transmitting TX surface coils for exciting a limited part of the examination region allows a low SAR to be achieved, but nevertheless has the serious disadvantage of not being able to generate a sufficiently homogeneous excitation field. The generation of an excitation field that is as homogeneous as possible is necessary, because only an MR image with a high contrast and a high quality can be generated as a result. Transmitting TX surface coils are therefore not often used in practice for exciting the examination object. From the point of view of the imaging sequence, an inhomogeneous excitation field results in a variation of the flip angle of the nuclear spins across the imaging volume, which in turn negatively influences the image contrast and the signal-to-noise ratio. It is desirable to obtain contrast that is as constant and well defined as possible at the same time as obtaining a signal-to-noise ratio that is as high as possible.
The use of spatially limited volume coils solves this problem, but only those parts of the examination object that can be completely enclosed by the volume coil can be examined in this way. Other parts, such as e.g. a shoulder, the hips or the spinal column, cannot be enclosed by volume coils, as a result of which the examination based on a spatially limited coil is significantly hampered.